1. Field of the Invention
The invention relates to a process for separating metal chlorides from their suspensions in chlorosilanes, which are obtained in the condensation of gaseous reaction mixtures comprising metal chlorides from the reaction of technical-grade silicon and hydrogen chloride, and to the further treatment of the metal chlorides to give a solution of metal chlorides in aqueous hydrochloric acid. The invention further relates to a filtration and dissolution apparatus for separating the suspension of metal chlorides in chlorosilanes and for the further treatment of the metal chlorides which have been separated off.
2. Discussion of the Background
It is known that technical-grade silicon, containing metallic impurities, can be reacted with hydrogen chloride at temperatures of from 270xc2x0 to 1,000xc2x0 C. both in fixed-bed reactors and in fluidized-bed reactors to form chlorosilanes. This process gives a gaseous reaction mixture which consists predominantly, depending on the reaction temperature, of a mixture of trichlorosilane and tetrachlorosilane (silicon tetrachloride). The metallic impurities in the silicon, mainly iron, aluminum and calcium, are converted into the corresponding chlorides. A part of these chlorides is carried from the reactor together with fine silicon dust whose amount varies greatly depending on the reactor type and throughput, and is deposited in cyclones or filtration units located downstream of the reactor. In particular, calcium chloride and iron chloride are predominantly deposited as solid metal chlorides on the fine silicon particles upon cooling of the gaseous reaction mixture, and can thus easily be carried out with the dust. The remainder of the metal chlorides, predominantly aluminum chloride, remains in vapor form in the cooled gaseous reaction mixture.
Aluminum chloride in particular tends to be deposited in solid form in pipes, on cooling surfaces or other equipment surfaces at temperatures below about 180xc2x0 C., which have to be employed for condensing the chlorosilanes under atmospheric pressure. The prior art therefore provides various solutions for carrying out the condensation of the chlorosilanes in a trouble-free manner despite this tendency for deposition of solid, and to separate off the aluminum chloride present as elegantly as possible. In a known process, a first condensation stage is carried out using indirect cooling, usually by means of cooling water as the cooling medium, in upright shell-and-tube heat exchangers through which the gaseous reaction mixture comprising metal chlorides flows from the bottom upward. The condensate which flows downward is supposed to flush away the metal chloride which is precipitated as a solid and keep the heat-exchange surfaces free. To avoid blockages at the gas inlet, xe2x80x9cknock-offxe2x80x9d devices are often installed at these particularly critical points so as to keep the flow cross-section free for as long as possible. The suspensions of metal chlorides in chlorosilanes formed by condensation have to be treated to separate off the metal chlorides.
In a further known process employing indirect cooling, the gaseous reaction mixture containing metal chlorides is passed through a simple double-walled condenser. This method also gives a suspension of metal chlorides in chlorosilanes from which the metal chlorides have to be separated off. However, part of the metal chlorides gradually forms a growing deposit on the cooling surfaces, as a result of which the flow cross-section is correspondingly reduced. When a particular fill level has been reached, the reaction mixture is switched over to a similar parallel condenser, the metal chloride deposit is removed by flushing with water and the condenser is subsequently dried. This operation is associated with frequent removal and reinstallation of the condenser, but still allows pseudocontinuous operation.
According to DE 629 853, the gaseous reaction mixture containing metal chlorides is passed into a melt which comprises an aluminum chloride/alkali metal chloride mixture and largely holds back aluminum chloride and iron chloride. The chlorosilanes are isolated by condensation of the vapors which are largely free of metal chlorides.
Finally, a continuous process is known in which the gaseous reaction mixture containing metal chlorides is passed into liquid chlorosilanes and the precipitated solid metal chlorides are separated from the liquid chlorosilanes. In a particular embodiment of this process, the gaseous reaction mixture containing metal chlorides (crude gas) having a temperature which may be about 300xc2x0 C., for example, is brought into intimate contact with a vertically directed, finely divided stream of chlorosilanes. The chlorosilanes are advantageously the reaction product of the chlorosilane synthesis, which is used, for example, at a temperature of from 40 to 50xc2x0 C. Part of the chlorosilanes (particularly the low-boiling trichlorosilane) is vaporized, the crude gas is appropriately cooled and the metal chlorides separate out as solids in the liquid chlorosilanes. The mixture of cooled crude gas and liquid chlorosilanes containing metal chlorides is passed to a separation vessel, from whose upper part the gas phase is taken off laden predominantly with relatively low-boiling chlorosilane vapors. From the bottom part of the separation vessel, part of the liquid chlorosilane phase enriched in the higher-boiling chlorosilanes and containing the suspended metal chlorides is taken off. The metal chlorides are separated off and the liquid phase is worked up by distillation to isolate the chlorosilanes. Another part of the liquid chlorosilane phase is introduced as runback into a column in which the entrained chlorosilanes are substantially scrubbed out from the abovementioned gas phase laden predominantly with relatively low-boiling chlorosilanes. From the remaining gas phase, the predominantly relatively low-boiling chlorosilanes, which are always still present therein in considerable amounts are condensed out by low-temperature cooling and part of them is worked up by distillation to isolate chlorosilanes; if desired together with the chlorosilane phase, if desired, which has been taken off from the lower part of the separation vessel and has been freed of metal chlorides, and the other part is returned as runback to the abovementioned column.
Most of the processes mentioned above give suspensions of metal chlorides in chlorosilanes from which the metal chlorides must be separated. The separation is made difficult by the fact that trichlorosilane is flammable in air and that all chlorosilanes are sensitive to hydrolysis. These problems also apply to the chlorosilanes that adhere to the metal chlorides which have been separated off. In addition, the hydration of aluminum chloride, which represents the main part of the metal chlorides, can occur in an explosive manner. Furthermore, for reasons of work safety, precautions must be taken to avoid emissions of chlorosilanes and hydrogen chloride. Although the process of DE 629 853 does not give suspensions of metal chlorides in chlorosilanes, it has the disadvantage that the melt frequently has to be replaced, for which purpose part of the circulated melt must be taken off continually or at frequent intervals and worked up.